The present invention relates to methods and constructions for releasable clip-on attaching of heat sinks to encapsulated semiconductor packages.
Heat is generated during the operation of integrated circuit devices. In some instances, the heat generated is severe enough to irreparably damage or even destroy the device unless it is dissipated to the surrounding environment. Continuing advances in the number of transistors and other functional elements contained in a single integrated circuit and the increasingly higher speeds at which integrated circuits now operate, both contribute to the problem of integrated circuit heat generation.
Different heat sink constructions have thus been developed for removing at least some of the heat from integrated circuit devices. These heat sinks often are unitary structures having a heat transferring portion proximate to the semiconductor device for extracting heat therefrom and a heat dissipating portion remote from the die and with a large surface for dissipating heat from the heat transferring portion. The dissipating portion is typically formed with a number of parallel (often vertical) layers, through which air passes or is passed to remove heat from the heat sink.
In other words, heat sinks are used to maintain the temperatures of semiconductor device junctions within acceptable limits. Unfortunately, there is no clean and easy way to attach heat sinks on encapsulated packages. Since the top surfaces of encapsulated packages do not have a uniform thickness, direct heat sink attachment without any modification in the heat sink base cannot be done as a practical matter.
One prior art method is to have a picture-framed recessed cavity in the heat sink base and to use epoxy to attach the heat sink to the encapsulant. Thereby the epoxy not only bonds the heat sink to the package, but also provides thermal enhancement by filling the heat sink cavity. In other words, the heat sink is directly epoxied to the top of the package. Another prior art method uses thermal grease to fill the cavity and then attaches the heat sink to the package with clips. However, using epoxy or thermal grease presents cleaning issues and therefore is not preferred for many applications.
The packages are mounted on a motherboard, typically by reflowing a solder ball array therebetween. They are mounted often with many other packages. When problems arise with one of the packages, that package usually needs to be removed from the underlying motherboard. This requires that heat be carefully directed to the solder balls under that package to reflow them. Special equipment is used to heat the solder balls, and this equipment is positioned on top of the package and either blows hot air or uses conduction heating. Since the heat sink has a high thermal capacitance, it is difficult to effectively heat and reflow the solder balls underneath the package with the heat sink mounted on the package, because the heat sink itself absorbs much of the applied heat. Thus, more heat needs to be applied, and this is time consuming and/or potentially injurious to the surrounding electronic components mounted on the same printed circuit board.
Alternatively, the heat sink can be removed from the package before the heat is applied to reflow the solder balls. However, this requires a means for attaching the heat sink to the top of the substrate such that the heat sink can be readily removed when desired. One such means is a spring clip which fits over the heat sink and onto edges of the substrate or the motherboard and has its tension adjusted by the user with a screw driver. Unfortunately, this may cause the sides of the package to bend up and warp.